One known anaerobic treatment method for organic wastewater is an upflow anaerobic sludge blanket (UASB) method, in which organic wastewater containing soluble BOD is subjected to high-load high-speed treatment by passing the organic wastewater through a sludge blanket of high-density settling granular sludge in a reaction vessel. In this method, a solid organic substance having a low digestion rate is treated separately, and only a soluble organic substance having a high digestion rate is subjected to high-load high-speed anaerobic treatment using granular sludge having a high anaerobic microorganism density. An expanded granular sludge blanket (EGSB) method is also known as an improved UASB method. In this method, wastewater is subjected to high-load anaerobic treatment at a high flow rate through a wide-spreading sludge blanket in a tall reaction vessel.
In anaerobic treatment using granular sludge, such as the UASB method or the EGSB method, granular sludge containing an anaerobic microorganism is grown. These methods can have a higher sludge concentration than a fixed bed or a fluidized bed, which contains sludge on a carrier, and can be used in high-load operation. These methods can be started up within a short time by obtaining surplus sludge from a treatment system in operation. Thus, these methods are among the most efficient anaerobic treatment methods.
These methods using granular sludge are very efficient when the COD concentration of wastewater is high (CODCr concentration of approximately 2000 mg/L or more). When the COD concentration is low (CODCr concentration of approximately 2000 mg/L or less), however, a large amount of water must be supplied to a reaction vessel, making granules to easily wash out from the reaction vessel. This tends to result in unstable performance.
When wastewater in which granules are difficult to form is treated by these methods, initial granules gradually decompose, and operation cannot be continued.
In contrast, in a method using a fluid non-biological carrier, the outflow of the carrier from a reaction vessel can be prevented by a mechanical method, such as a screen. Furthermore, the carrier surface can always serve as a growing medium for a microorganism. Thus, a method using a fluid non-biological carrier can advantageously be applied to wastewater of a low concentration COD or wastewater in which granules are likely to decompose.
A non-biological carrier has a high degree of freedom of design, such as specific density or size, and allows a much higher settling velocity than granules. Use of a non-biological carrier having a high settling velocity can obviate the necessity for a solid-liquid separation mechanism (GSS), which is required for granule methods, increase the effective volume of a reaction vessel, and significantly reduce construction costs.
In a method using a fluid non-biological carrier, however, a microorganism adheres to a carrier to form a biofilm on the carrier surface and generates gas within the biofilm. The gas adheres to the carrier. This reduces the apparent specific density of the carrier and floats the carrier in a reaction vessel, and the carrier flows out of the reaction vessel together with treated water. Such a problem can be solved by using a carrier having a large specific gravity and a high settling velocity. However, an anaerobic treatment method using a carrier having an excessively large specific gravity and an excessively high settling velocity has low contact efficiency between the carrier and water to be treated and low treatment efficiency. Furthermore, a solid deposited on a layer of settled carrier can block the flow path.
In one proposed method to prevent the flotation of a carrier, a reaction vessel is agitated with impeller blades to remove air bubbles from the carrier using a swirl flow, thereby recovering the settleability of the carrier. However, the carrier may be broken by a collision with the impeller blades.
Patent Documents 1 and 2 describe an apparatus for extracting a floating carrier from a reaction vessel and returning the carrier to the reaction vessel through an external circulation line.
Air bubbles adhering to the floating carrier extracted from the reaction vessel can be removed while the carrier returns to the reaction vessel through the circulation line. However, the present inventors found that only the circulation of a carrier without consideration of the settleability of the carrier cannot solve the problems of the flotation and adhesion of the carrier in a reaction vessel. Furthermore, since not much consideration has been given to the relationship between the settleability of a carrier and circulation means, the circulation requires a complicated mechanism and high maintenance costs. For example, Patent Document 1 describes a complicated collection and reflux line and a liquid jet mechanism for the circulation of a carrier. Such a mechanism cannot be easily attached to an existing reaction vessel, is likely to break down, and requires complicated maintenance. Patent Document 2 describes an ejector mechanism in a circulation line of a carrier. Such a mechanism also has similar problems.
Various fluid non-biological carriers are used in such treatment. For example, Patent Document 3 describes use of a carrier having a particle size in the range of 0.1 to 0.3 mm and a specific gravity in the range of approximately 1 to 3. Patent Document 4 describes use of granular organic gel fine particles having a particle size in the range of 0.5 to 0.6 mm as a carrier. The gel fine particles swollen with water have a specific density in the range of 1.00 to 1.50, preferably 1.01 to 1.10. The settling velocity of the gel fine particles in connection with the present invention may be in the range of 15 to 150 m/h.
Patent Document 5 describes “a fluid treatment carrier, comprising a foam containing a polyolefin resin constituting 30% to 95% by weight and a hydrophilizing agent of a cellulose powder constituting 5% to 70% by weight, the foam having surface melt fracture” and “a fluid treatment carrier, comprising a foam containing a polyolefin resin constituting 30% to 95% by weight, a hydrophilizing agent of a cellulose powder constituting 4% to 69% by weight, and an inorganic powder constituting 1% to 30% by weight, the foam having surface melt fracture” as fluid treatment carriers of a polyolefin foam having excellent fluid treatment performance, such as settleability in water. Patent Document 5 also describes denitrification treatment evaluation results using the fluid treatment carriers. However, the size and the settling velocity of the carriers are not clear, and a specific treatment method is not described.
In anaerobic treatment using a fluid non-biological carrier, a low settling velocity of the carrier results in poor settleability and the flotation and outflow of the carrier after the formation of a biofilm on the carrier surface. On the other hand, a high settling velocity of the carrier results in poor flowability and adhesion and blockage due to a grown biofilm, making the operation impossible.
The surface properties of a carrier greatly affect the number of microorganisms adhering to the carrier and the treatment capacity of a reaction vessel. A decrease in the number of microorganisms adhering to the carrier to maintain settleability results in a low treatment capacity. On the other hand, when a microorganism can grow to the inside of the carrier in order to increase the number of microorganisms adhering to the carrier, the biofilm becomes thick, and the carrier floats because of air bubbles formed within the carrier.
UASB and EGSB using granules are principally applied to high-concentration wastewater and are generally not applied to low-concentration wastewater having a CODCr concentration of approximately 2000 mg/L or less. This is because granules may wash out of a reaction vessel, and the outflow of granules in low-concentration wastewater often exceeds the number of granules grown in the reaction vessel, making it difficult to hold the granules in the reaction vessel for a long period of time.
A method using a fixed bed or fluidized bed carrier rather than granules has been applied to low-concentration wastewater. In the case of the fixed bed carrier, a support bed for holding a biofilm is fixed in a reaction vessel, and a microorganism grown on the surface of the support bed is used. In the case of the fluidized bed carrier, a carrier having a controlled specific density and size is fluidized in a reaction vessel, and a biofilm formed on the carrier surface is used for treatment.
Irrespective of fixed bed carriers or fluidized bed carriers, use of non-biological carriers disadvantageously takes time for a microorganism to adhere to the carriers and takes considerable time to start up apparatuses. Hitherto, in the startup of an apparatus, a reaction vessel containing a carrier has been charged with dispersed seed sludge, and the flow of water has been limited in order to prevent the outflow of the seed sludge. Normal operation is started after a biofilm is formed on the carrier surface. As described below in Comparative Example 1, however, such procedures take a long time of 90 days before the startup of the apparatus.
Patent Document 6 proposes a mix-bed reaction vessel of a carrier and granules. The reaction vessel described in Patent Document 6 always includes a mix-bed of the carrier and the granules and must operate under limited conditions where the granules in the reaction vessel are not dispersed.
Patent Document 7 discloses that sludge flowing out of a reaction vessel during the startup of an apparatus in treatment by the UASB method is adsorbed on a carrier in a carrier column vessel installed downstream from the reaction vessel and is returned to the reaction vessel. However, this is not intended to promote the adhesion of a microorganism to a carrier in the reaction vessel during the startup of the apparatus.
Patent Document 8 discloses that water-absorbing polymer hydrogel particles in a blanket in treatment by the UASB method promotes the formation of granules during the startup. This method is also not intended to promote the adhesion of a microorganism to a carrier in a method using a fixed bed or fluidized bed carrier.
As described in an example of Patent Document 9, granular sludge obtained from an UASB reaction vessel of another system is supplied as seed sludge at the startup of a reaction vessel in the UASB method. In a method using a fixed bed or fluidized bed carrier, however, granular sludge is not supplied at the startup of a reaction vessel.